Thus far, as a method for measuring human cerebral function, there has been a technique for measuring activities of a cerebral cortex through the use of light.
In bio-instrumentation using light, a device for measuring a living body function using visible light to near infrared light has been disclosed in, for example, Patent Document 1, Japanese Patent Laid-Open No. 115232/1982, or Patent Document 2, Japanese Patent Laid-Open No. 275323/1988. Further, an invention concerning an image measurement technique for a cerebral function which uses the present principle of measurement has been disclosed in Patent Document 3, Japanese Patent Laid-Open No. 1997/98972.
These use such light wave-guides as represented by optical fiber or the like to converge light (hereinafter abbreviated as living body passage light) which by irradiating light on a living body, has permeated through at a position apart by several mm to several cm while being scattered within the living body for measuring. From intensity of the living body passage light measured, such a concentration of photoabsorption substance within the living body as represented by oxygenation hemoglobin, deoxidation hemoglobin and the like or a value corresponding to the concentration will be determined. When determining the concentration of photoabsorption substance or the value corresponding to the concentration, a photoabsorption characteristic of the photoabsorption substance aimed at, corresponding to wave length of the light irradiated will be used. Generally, when measuring the depth of the living body from the surface of the living body, there will be used light having wave length within a range of 650 nm to 1300 nm which has high living body permeability.